1. Field of the Invention
The present invention relates to protection circuits and, in particular, to circuits for protecting NMOS devices from excessive voltage stress.
2. Description of the Prior Art
In recent years, the technology of integrated circuits has been improved rapidly, and the size of CMOS transistors has been continuously diminished. To conform to the characteristic of small-sized components and reduce the power consumed by CMOS transistors, generally, the voltage level of power provided to the transistors is also decreased along with the size of CMOS transistors.
However, IC chips manufactured in early years still use higher power voltages (e.g. 5V). To cooperate with circuits using higher power voltages, circuits using lower power voltages (e.g. 3.3V or 1.8V) must include interface circuits with special configurations. Please refer to FIG. 1, which illustrates an exemplary interface circuit in the prior art.
As shown in FIG. 1, a cascoding NMOS transistor (NC) is coupled between an NMOS device (ND) and an external voltage source (VEXT). The gate of NC is usually coupled to an internal voltage source (VINT) of the IC which includes NC. The function of NC is to provide a cross voltage between its drain and source, and thereby ND does not have to directly endure the high voltage stress induced by the external voltage source VEXT.
For instance, the voltages of the external voltage source VEXT and the internal voltage source VINT are assumed as 5V and 3.3V, respectively. The voltage at the source of NC is typically lower than that of at its gate; their voltage difference is the threshold voltage of NC. Therefore, when the gate voltage of NC is 3.3V, the source voltage of NC will substantially be 2.3V. Under this condition, the cross voltage between the drain and source of NC is 2.7V. Because this cross voltage is in an endurable range, NC will not be damaged by excessive voltage stress and can provide protection for ND.
However, if the voltage of the external voltage source VEXT is 5V and the voltage of the internal voltage source VINT is reduced to 1.8V, the voltage at the source of NC will be approximately 1V. Hence, the cross voltage between the drain and source of NC will be 4V. Under this condition, NC will probably be damaged by the cross voltage and lose the function of ND protection.